1. Field of the Invention
The present invention relates to floating power generation systems and relates particularly but not exclusively to such systems having stabilization control.
2. Discussion of Prior Art
Offshore power generation which harnesses the power of the waves has the potential to become a major source of energy and has been the subject of much experimentation and development over the past forty or more years. Such systems harness the power of the waves either directly or indirectly and convert that energy into electricity which is then transported to shore for subsequent use. The oscillating water column (OWC) has become a very popular method of converting wave energy into electrical power, whether as a shore-based, bottom-mounted or floating device. Whilst there are many ways of harnessing the wave energy, virtually every OWC proposed or built in the last 20 years has one or more Wells turbines which are driven by the pressurised air escaping from or entering the column as the water rushes in and out thereof. The popularity of the OWC has a great deal to do with the convenience with which the Wells turbine converts bi-directional airflows between wave chamber and atmosphere into unidirectional bursts of torque in the coupling of the electrical generator. Moreover, during lulls in the sea or when the air velocity drops to zero during twice-per-wave flow reversal, the Wells turbine needs little power to stay rotating. Simpler Wells turbines employ a set of fixed pitch blades and whilst these provide a generally very positive contribution to the creation of electrical energy the range of wave, and therefore airflow, conditions over which a fixed blade Wells turbine operates with reasonable efficiency is severely limited by blade stall.
Typically, Wells turbines use symmetrical profile blades with their chords in the plane of rotation and often produce positive torque only for angles of incidence between 2 and 13 degrees. Below 2 degrees, in the low air velocity operating area, the lift component is too small to produce positive torque and the rotor tends to lose speed. At angles of incidence above 13 degrees the blade section stalls. The rapidly increasing drag forces dominate the less rapidly declining lift forces and efficiency is compromised. If, however, the blades are such as to be able to change pitch so as to prevent the angle of incidence exceeding some maximum angle, for example 8 degrees, then it would produce positive torque at all angles of incidence above 2 degrees and efficiency would improve.
In operation, the water level oscillates up and down within the water column as the crests and troughs of the waves pass through the water column. If this oscillating water level is made to take place in a structural column opened at both ends, the air column above the water oscillates in a similar manner and, thus, wave energy is thereby converted into low pressure, high volume air flow. Energy is then extracted from the moving air by a self-rectifying Wells turbine, in which rotation is unidirectional regardless in which axial direction air is flowing. In essence, the Wells turbine is essentially operated as a wind or aero turbine. The working interface is therefore between water and air, and air and rotor blades. The turbine reacts to the low pressure air stream which is far less destructive than directly absorbing the powerful impact force of sea waves. The efficiency of energy transfer between the wave and the air is high if not total whilst the energy transfer efficiency at the air/rotor blade is very much dependent upon good design and efficient management of the energy transfer itself.
In some arrangements it is known to use a flywheel to keep the turbine spinning by virtue of momentum during times when the waves are weak. It is also known to use two rotors in tandem configuration that rotate in opposite directions and are coupled to a common output. It is also known to use the variable-pitch turbine for performance-enhancing reactive loading by using the generator and turbine to pump bursts of energy into the wave chamber itself.
It is also well known to extract energy from the wind by causing the wind to drive a wind turbine which is, typically, mounted as high on the platform as possible so as to ensure it is exposed to the full force of the wind whilst being clear of any ground effect or interference created by the platform itself. Whilst such turbines are relatively efficient and are able to extract large amounts of energy from the wind, the higher efficiency turbines tend to have very large diameter blades and hence require very high platforms or towers upon which they can be safely mounted. On land this does not present a problem as the tower can be firmly secured to ground but the security of fixture is somewhat more problematic when mounted to a floating platform which is subjected to the motion of the waves. Any such motion causes the turbine to oscillate from a steady state condition and creates what can be adverse structural loadings on both the turbine and the support tower itself. Some water based wind turbine arrangements are operated to take advantage of the forward and backward motion caused by the waves interacting with the platform or floating column upon which it is mounted. In essence, forward motion creates an “apparent wind” to which the rotor blades are exposed and more energy can be extracted from the wind during any forward motion of the blades than can be extracted when the blades are either stationary or being rocked backwards. Whilst this additional energy extraction can be advantageous it has to be balanced against the structural loading on the support tower upon which the turbine is mounted and this can be undesirably high in stormy sea conditions and may lead to structural failure.
In addition to the above-mentioned problems, such platforms also suffer from adverse movement in up to six axes (roll, yaw, pitch, heave, sway and surge) whilst floating on what can be very choppy seas. Movement in any one or more axis will have an adverse affect on platform stability, structural loading and also power generation and is preferably reduced to a minimum in order to prolong platform life and energy extraction. Various systems have been proposed to stabilize the platform itself, one of which is discussed in EP 0053458 in which the column of water in an OWC is arrested and released subsequently in an attempt at synchronizing. Whilst the above arrangements provide very reasonable solutions the power generation or stability problems, maximizing power generation can sometimes be to the detriment of structural loading or stability whilst maximizing stability can have an adverse effect on power generation.
Yaw control is particularly important when attempting to stabilize a moored platform at sea and is not readily addressed by the above-mentioned arrangements. Often the waves are of such power, magnitude and direction as to sway the platform around on its moorings which are then placed under additional strain which can be substantial. When mooring lines are also provided they can exert a corrective force on the platform thus sending the platform into an oscillating motion which can be difficult to control and can sometimes be of a frequency that matches another external force which when combined with the yaw force places the platform under excessive load. Some platforms are designed to face into the oncoming waves and are shaped such as to provide a bow or other such feature but when such features are not present such platforms can become unstable in high sea states and this can also lead to severe strain being placed on the platform itself and any wind turbine structures placed thereon. This problem is exacerbated by wind/tide conditions which place the wind at an angle relative to the oncoming wave.